The most studied metallocenes to date are titanium and zirconium based mono- and bis-cyclopentadienyl metallocenes. These catalysts, in particular, zirconocenes, are known to produce low density polyethylenes—those having a density of about less than 0.930 g/cm3—at an advantageously high activity under many types of reactor conditions. However, it is known that the most studied metallocenes to date do not produce higher density polyethylenes—those whose density is above 0.930 g/cm3—at a productivity that is commercially acceptable. See, e.g., 1 METALLOCENE-BASED POLYOLEFINS 12–14 (J. Scheirs & W. Kaminsky, eds., John Wiley & Sons 2000). Furthermore, it is known that hydrogen reduces the productivity of most metallocenes. See, e.g., J. B. P. Soares and A. E. Hamielec in 3(2) POLYMER REACTION ENGINEERING 131–200 (1995). This is a disadvantage in that hydrogen is useful (and most often necessary) for increasing the melt index (I2) of the polyethylene being produced, higher I2 (e.g., 5–50 dg/min) resins being desirable for such applications as rotomolding and injection molding products. It is also a disadvantage in that it would be desirable to use the same catalyst composition when transitioning between high, medium and low density polyethylene products in a single or multiple reactor(s), and avoiding the current need of having to change catalyst composition in order to achieve different resin products.
Metallocene derived resins have many advantageous properties that provide commercially attractive products. What would be desirable is a metallocene catalyst composition that is capable of producing medium and high density polyethylenes at commercially attractive productivities, especially polyethylenes useful in rotomolding and injection molding applications. Further, it would be useful to provide a metallocene that is capable of being used to produce a wide melt index and wide density range of polyethylene resins at commercially attractive productivities. The inventors here have discovered such a catalyst composition and process for producing these resins.